introduce simple CallbackHandle
A CallbackHandle is used to cancel a previosuly submitted future with set callback without keeping the future object around. A CallbackHandle remembers the callback pointer and the IoContext where the callback was submitted to submit a cancel operation for the pointer on the same IoContext. A CallbackHandle can be obtained by calling Future::submitAndGetCallbackHandle().
cancel-callbacks-simple in 16 minutes and 31 seconds (queued for 5 seconds)50 jobs for